1. Field of the Invention
This invention relates to supplying control signals to the output devices in a totem-pole or bridge connection where a common reference cannot be maintained between the control signal source and the input terminals of the output devices.
2. Description of the Related Art
Totem-pole connections of power transistors have been used in output driver circuits of supply power to a load device. Each totem-pole circuit is formed by a series connection of a pair of power devices. A bridge circuit can be formed by the combination of a pair of totem-pole drivers so that the load device can float between the outputs of the two totem-pole circuits and not require a direct connection to one of the supply terminals.
A common problem with using totem-pole or bridge circuits of this type is that it is difficult to provide control signals to the power devices due to the lack of a single common reference connection to each of the power devices in the circuit. Several solutions to this problem are presented in the literature, the most common of which is to use a transformer to couple the appropriate signals to the input terminals of the power devices. Of course, the desireable characteristic of the transformer is its ability to couple signals between its input terminals and its output terminals without having a wired connection between the input terminals and the output terminals. The coupling takes place because of the magnetic fields shared between the input and output connections.
The use of transformers to couple the signals to the input terminals of the power devices does have some disadvantages. If the signal being coupled to the output devices contains significant low frequency energy, the transformers are larger and heavy, and are generally of the iron-core variety. Further, transformers cannot couple dc signals, and the leakage inductance of the transformer can cause problems, such as overshoot in the output signal.
Another method to provide control signals to the output devices of totem-pole or bridge circuits is to optically couple the signal from a light emitting diode (LED) to a photodetecting transistor at the input terminals of the output devices. The LED-phototransistor method of coupling the signal also has problems as it is presently shown in the literature. First, the LED-phototransistor coupler is slow, and limits the maximum possible switching speed of the output circuit. This can be a significant restriction for the use of this method in Pulse Width Modulation output circuits. Another limitation of the LED-phototransistor coupler is the sensitivity of the base terminal of the phototransistor. The base terminal of the phototransistor has a high current gain (the beta, or hfe of the transistor), and as a result, signals which get parasitically coupled to this node (such as the inherent parasitic capacitances) can get amplified significantly. This amplified signal due to the parasitic coupling is undesired, and causes distortion in the output signal.
A more complete discussion of methods to couple signals to output devices appears in the Motorola Data Book Power MOSFET Transistor Data, Chapter 2-6, "Gate Drive Requirements," Q2/89, DL135, REV 3, to which the reader is referred for background and a more detailed analysis.